1. Field of Invention
The invention relates generally to imaging systems and more particularly to imaging systems for use in nuclear medicine.
2. Description of Related Art
In a nuclear medicine imaging device, such as a gamma camera for obtaining either planar images or Single Photon Emission Computed Tomography (SPECT) images, a collimator is mounted to the face of the imaging device. The collimator is constructed of a dense, high-atomic-number material, such as lead. The material is bored with numerous tiny straight holes that allow radiation (e.g., gamma rays) to pass through. If the radiation is not traveling along the path of the hole, then the material absorbs it and it will not reach the detector. The collimator thus collimates radiation, which is emitted from a distributed source (e.g., a radiopharmaceutical or radioisotope chosen for its affinity for a particular organ, tissue or region of the body) within a patient, before the radiation strikes a detector crystal.
FIG. 2 is a block diagram of an exemplary SPECT or planar imaging device. A radiation source 302 within an object to be imaged 304 (e.g., human body part) emits gamma photons that emanate from the object 304, pass through the collimator 308, and are captured by a detector 306, usually a large flat crystal of sodium iodide with thallium doping in a light-sealed housing, that converts the detected radiation into spatial projection data. The system accumulates counts of gamma photons that are absorbed by the crystal in the detector 306. The crystal scintillates in response to incident gamma radiation. When an absorbed gamma photon releases energy, it produces a faint flash of light. This phenomenon is similar to the photoelectric effect. Photomultiplier tubes (PMT) behind the crystal detect the fluorescent flashes and convert them into electrical signals, and a computer 310 sums the fluorescent counts. The computer 310 in turn constructs and displays a two dimensional image of the relative spatial count density or distribution on a monitor. This image then reflects the distribution and relative concentration of radioactive tracer elements present in the organs and tissues imaged. The two dimensional images are also referred to as planar images because they are taken from only one angle and are similar to an x-ray radiograph.
In order to obtain spatial information about the gamma emissions from an imaging object, a method of correlating the detected photons with their point of origin is required. Single Photon Emission Computed Tomography (SPECT) captures multiple images from multiple angles in order to reconstruct a three-dimensional representation of the region of interest (ROI). SPECT is usually performed using a parallel-hole collimator. The parallel-hole collimator does not provide any depth information as to the spatial origin of a gamma incident on its face. Thus, reconstructing the image in three dimensions requires processing multiple planar images of the ROI from multiple view angles in a manner well known in the art to yield a human-readable, three-dimensional image of the object. Because of the need to acquire planar images from multiple view angles sufficient to reconstruct tomographic images, the required scan time is relatively long.
A varying focal-length or multi-focal collimator (MFC) has multiple focal points for axial and transaxial detector directions. The advantage of a MFC is that it enables the imaging of small organs or regions within a field of view (FOV) of a gamma camera detector to be imaged faster than with use of a parallel-hole collimator, as the acquired projection data can be limited to the ROI within the larger FOV. Tomographic reconstruction methods for MFC acquired projection data are known. However, such methods use such projection data in the same way that projection data are used in parallel-hole collimator imaging.
In particular, MFC reconstruction methods fail to take into account that the Point Spread Function (PSF) of a detector with a MFC is non-stationary with respect to the position of the source relative to the MFC collimator surface, in contrast with parallel-hole collimators, where the PSF is static with respect to the source position relative to the collimator surface (i.e., in the axial-transaxial direction). The PSF describes the response of the detector to a point source of radiation. In a parallel hole collimator, the PSF of the detector is static with respect to the location of a point source vis-a-vis the collimator. In contrast, the PSF of a non-parallel hole collimator is non-stationary, meaning that the PSF of an MFC detector varies as the test point source is shifted in position with respect to the collimator, in all directions.
Thus, the possibility exists for improvement in image quality by taking into account the depth information available from consideration of the fact that PSF in a MFC is non-stationary.